Osteoporosis (bone thinning) occurs in a large percentage of individuals with Cystic Fibrosis (CF) and may lead to fractures. Many individuals with CF have bone density similar to the average 60-70 year old person. As CF patients live longer, fractures may compromise their life styles by causing pain and debilitation. The purpose of this research study is to determine if alendronate, a drug possibly useful in the treatment of osteoporosis, is sufficiently well absorbed in patients with CF. Ultimately, the purpose of this research is to determine if osteoporosis in cystic fibrosis patients can be reversed, at least in part, by vitamin D and calcium with or without alendronate. If these therapies are effective, less fractures will occur and quality of life will be improved.